A two-step prior art synchronization method of the generic type is known from DE 199 61 557 A1.
Before setting up a data link to one or more base stations, every mobile radio receiver must synchronize itself to a transmission and reception clock. For this purpose, the base station generally periodically transmits a specific signal, which comprises a predetermined sequence of chips, is known to the receiver and is referred to in the following text as a “sync word” or as a time slot synchronization code. This signal is looked for in the incoming data stream in the receiver. This procedure is carried out automatically in the mobile radio receiver, on the basis of the software programming of its hardware structure.
The data stream is subdivided into so-called time slots (or slots) with a fixed number of bits. At the start of a time slot such as this, there is a sync word, which must be identified by means of a suitable method. Furthermore, a specific number of successive time slots in the data stream are combined to form a frame.
Two different solution approaches are known from the prior art for identification of the time slot start.
In the first solution approach that is known from the prior art, the received signal is correlated with a sync word. To do this, the received data bits (corresponding to the length of the sync word) are taken from after a specific time, and a correlation with the sync word is carried out. The result is stored. This process is repeated until all of the potentially possible time slot starts in a time slot have been tested. The time with the maximum correlation value then corresponds to the time slot start.
In a second solution that is known from the prior art, the received data stream is filtered by means of a matched filter (FIR filter). The impulse response of this filter corresponds to the complex-conjugate sync word, mirrored in the time domain. The matched filter produces a result for each received bit. The maximum value at the filter output occurs when the time slot start has been found (minus the latency of the filter). This solution is described, for example, in: B. B. Ibrahim, H. Aghvami; IEEE Journal on Selected Areas in Communications; Vol. 12, No. 5; June 1994.
Since the mobile radio channel is not a steady-state channel, it is not sufficient to carry out the methods described above for only a single time slot. Typically, a number of time slots must be processed in order to come to a virtually error-free decision about the time slot boundary. In consequence, it is necessary to store all of the intermediate results. In a CDMA (Code Division Multiple Access) system, as is used for UMTS, the time slot length corresponds to 2560 samples (in each case with an in-phase component and a quadrature component), or to a multiple of this, if oversampling is carried out. An oversampling factor of 2 is normally used. Up to 10,240 samples, each typically comprising 8 bits, are thus stored. A corresponding RAM of 80 kbits would thus have to be provided.
The last-mentioned approach, in which averaging is carried out over all 2560 signal sample values multiplied by the oversampling factor OSF, is the standard approach used in the prior art to solve the time slot synchronization problem.
The standard synchronization method which is known from the prior art and whose result is very accurate has the disadvantage, however, that the amount of memory space required is very large and the power consumption is very high (computation operations). The two-step method which is known from DE 199 61 557 A1 filters the received data by means of a grid of a number of windows with the same rigid width and selection of the window with the greatest recorded energy in the first processing step reduces the amount of memory space required, but this prior art solution is at the expense of a considerable reduction in the recording accuracy. The reason for the poor accuracy of the method according to DE 199 61 557 A1 is that this method does not distinguish between information energy and disturbance energy in the first method step, so that, even if the disturbance level is low, a window which currently does not contain the sought time slot boundary, that is required for synchronization, is relatively frequently selected in the first method step for further processing.